The invention relates to a process for the purification of granular crystalline or amorphous silicon dioxide in a chamber at temperatures ranging between 700 and 1,300.degree. C. while a gaseous atmosphere of chlorine and/or hydrogen chloride gas is passed through the chamber and the silicon dioxide grains are thoroughly mixed during the cleaning process.
A process of the above kind is known from the British patent No. 834 383. At the treatment temperature, the impurities contained in the granular silicon dioxide diffuse to the particle surface where they react with the gaseous atmosphere so as to become a volatile compound of chlorine which, in turn, can be removed from the chamber in the form of vapors. This process permits reducing the impurity contents of the silicon dioxide grains, especially with regard to the elements aluminum, copper, iron, nickel, molybdenum, and antimony; however, it is not suitable to reduce the contents of the alkali elements in the silicon dioxide grains, especially the lithium, which has unfavorable effects in semiconductor technology. Impurity concentrations of less than 1 ppm of an alkali element cannot be achieved with this process.
GB-OS No. 21 66 434 discloses a cleaning process for articles made of quartz glass so as to reduce the contents of alkali impurities in these articles. In this process a constant (or steady) electric field between 100 V/cm and 10,000 V/cm is generated between two opposing surfaces of the article which is heated up to 700 to 2,000.degree. C. Under these conditions the alkali ions travel in the direction of the constant (or steady electric) field in the area of the one surface of the quartz glass article. This portion having alkali impurity deposits is then removed for example by means of etching with hydrofluoric acid or grinding. The treatment time, i.e., the retention time of the heated quartz glass article in the electric field, depends on the temperature at which this article is heated. For very high temperatures this time covers a period of several minutes; for lower temperatures the time range covers approximately some tenths of an hour, for example 40 minutes. This process permits treating quartz glass articles, especially pipes or crucibles, the alkali impurity concentration of which could be reduced to values ranging to 0.1 ppm. Granular amorphous silicon dioxide has been recovered by further manufacturing into powder pipes which were cleaned according to this process. Such a process for manufacturing purified granular silicon dioxide consumes a very high amount of energy. Furthermore, it cannot be excluded that impurities enter the final product while shattering the cleaned pipe and grinding up the shattered parts to obtain granular silicon dioxide. The electrodes for generating the electric field must be adjusted in form and size as far as possible to the quartz glass articles to be treated.